The present invention relates to television signal processing. More particularly, the present invention relates to improved signal processing in which both flicker and perceived line structure in the displayed television signal are reduced while minimizing undesirable motion artifacts and allowing relatively low horizontal scanning rates suitable for mass market television displays.
The 50 Hz, 625 line PAL and SECAM television systems were developed some forty years ago. Those systems have a 50 Hz field rate (312.5 lines per interlaced field) and a 25 Hz frame rate (625 interlaced lines per frame, only 576 of which are visible). At the time of their development, technology did not support large display sizes. Consequently, 576 lines per frame were adequate to render the line structure unnoticeable at normal viewing distances. Also, at that time, display brightness was much lower than in typical displays of the present era. As a result, flicker resulting from the relatively low 50 Hz field rate (refresh rate) was not a serious problem. When PAL and SECAM signals are displayed on today""s large screen television equipment, both flicker and line structure are visible and annoying to most viewers.
The problem of flicker is particularly objectionable in 50 Hz systems displayed on large bright screens. To overcome flicker, video systems have been produced that double the frame rate. However, frame doubling still leaves the line structure visible in large displays. Visible line structure is reduced by line doubling. One type of line doubling converts the interlaced signal to a progressively scanned one in which the progressively scanned frame rate is the same as the interlaced field rate and the progressive frames have twice as many lines as an interlaced field. Another type of line doubling maintains interlacing but doubles the number of lines in each interlaced field.
Some xe2x80x9chigh endxe2x80x9d television display enhancement products include both line doubling and frame doubling, including products manufactured by Faroudja Laboratories, Inc. of Sunnyvale, Calif. While providing an excellent picture display without flicker and visible line structure at normal viewing distances, such systems require a high horizontal scan rate. The Faroudja Laboratories product that provides a line doubled and frame doubled progressively scanned output requires a 63 kHz horizontal scan rate, a rate well above the performance capability of all by the best quality and most expensive display systems marketed in relatively small quantities. For the general market, television set manufacturers would like for horizontal scan rates to be below 40 kHz and preferably around 31 kHz for low cost display manufacturing. Thus, a line doubled and frame doubled combination would not be viable in mass market consumer applications. It would be desirable to reduce flicker and perceived line structure, while not requiring an increase in the horizontal scan rate above that supportable by mass market horizontal scanning systems.
In accordance with the teachings of the present invention, a method is provided for deriving an interlaced television signal from an interlaced 625 line, nominally 50 Hz field rate television signal, such as a PAL or SECAM television signal, the derived television signal having perceived reduced line structure and perceived reduced flicker, in which, in either order: (1) the field rate of the derived television signal is increased with respect to the field rate of the original television signal, such that the increase in field rate reduces perceived flicker in the derived television signal, and (2) the number of lines in each field of the derived television signal with respect to the number of lines in each field of the original television signal is increased, such that the increase in lines reduces perceived line structure in the derived television signal, wherein the increase in the field rate and the increase in the number of lines in the derived television signal results in a horizontal scanning rate that does not substantially exceed twice the horizontal scanning rate of the original television signal while minimizing undesirable motion artifacts.
The invention may be implemented in one of two basic ways. In a first approach, the number of lines in each field of the derived television signal is increased prior to increasing the field rate of the derived television signal. The number of lines in each field of the derived television signal are increased by de-interlacing the original television signal to produce a progressively scanned signal and then increasing the number of lines in each frame of the progressively scanned television signal. Alternatively and less desirably, the number of lines in the original interlaced signal may be increased followed by conversion of the line increased interlaced signal to a progressively scanned format. Then the field rate of the derived television signal is increased by reinterlacing the progressively scanned television signal such that for some progressively scanned frames a pair of interlaced fields are derived and for selected progressively scanned frames only one interlaced field is derived, thus dropping selected ones of the potential interlaced fields in order to compensate for the increased number of lines. Consequently, the required horizontal scanning rate remains relatively unchanged or is not substantially increased from the horizontal scanning rate required for a signal that is only line doubled or only frame doubled. The resulting line increased and frame increased signal is thus supported by low cost horizontal scanning systems.
In the second approach, the field rate of the derived television signal is increased prior to increasing the number of lines in each field of the derived television signal. This approach has the advantage of requiring fewer memory resources, thus reducing the cost of a practical implementation. This approach also readily lends itself to implementation in a practical implementation that is also operable to provide either line doubling by itself or frame doubling by itself. The field rate of the derived television signal is increased by deriving two or three signal streams from the original television signal. Each of the signal streams comprises a pattern of n repeated time-compressed fields (where xe2x80x9cnxe2x80x9d is a whole positive integer), each of said signal streams having a field rate substantially equal to the increased field rate, all of fields in a signal stream being of the same parity, at least one signal stream consisting of even parity fields, each field in a signal stream being substantially identical in information content to each consecutive field of the same parity in the original television signal. Two further signal streams are derived from the initial two or three signal streams by alternately selecting even and odd fields from the two or three signal streams for the first further signal stream, and by alternately selecting, from the two or three signal streams, a field of opposite parity to the field selected for the second further signal stream. Selected scan lines from the second further signal stream are added to the scan lines in the first further signal stream in order to increase the number of lines in the derived television signal.